"Our preliminary genetic studies suggest over the last 40 years or so, the selection pressure of pollution has driven the rainbowfish population towards higher copper tolerance," said researcher Ms Sharyn Gale.

The East Finnis River was heavily polluted by metals and acid draining from the Rum Jungle copper and uranium mine which opened in 1954. Even after its closure in 1971, acid drainage remained a serious problem until the leaking waste dumps were capped in the 1980s.

During the 1990s, although the majority of fish were killed in the first flush of heavy metals downstream at the start of each wet season, schools of the black-banded rainbowfish, Melanotaenia nigrans, were observed in the river. At the same time, copper levels recorded were nine times greater than the lethal limit for rainbowfish in other river systems.

Ms Gale and colleagues compared rainbowfish in the East Finnis River with the same species of rainbowfish in Coomalie Creek, an unpolluted catchment nearby. They found "polluted" fish to be eight times more tolerant to copper than "unpolluted" fish.

The team used radioactive isotopes of copper, 64Cu and 67Cu, as "tracers" to follow where and how much copper was concentrated in the fish. They found the gills of East Finnis River rainbowfish had adapted to reduce or block the amount of copper absorbed, enabling them to halve the amount of copper "bioconcentrated" in their bodies, compared to rainbowfish elsewhere.

The allozymes (or enzymes linked to particular genes) of the "polluted" and "unpolluted" fish were also compared in a process using an electrical charge to separate out the different allozymes.

These tests showed the East Finnis River rainbowfish have less genetic diversity than others, suggesting selection pressure is "pushing" the fish population towards more specific traits  such as selecting genes that create enzymes less sensitive to copper.

"However, the fish may have adapted this particular trait at the expense of other traits important for their survival, such as their ability to cope with pesticides, turbidity, temperature, or salinity," said Ms Gale.

Ms Gale points out this is only the second recorded case in the world of freshwater fish adapting to heavy metal pollution, and as such raises further questions.

"To see evolution in action, we need to see copper tolerance passed down to succeeding generations of rainbowfish; to test the enzymes to see how they produce tolerance and to find out how the gills actually block copper absorption," she said.